01 - Preheat oven to 350°F. Generously grease a 9x13-inch baking pan with butter or cooking spray to prevent sticking.
02 - In a large mixing bowl, combine flour, granulated sugar, softened butter, milk, eggs, baking powder, salt, and vanilla extract. Beat with an electric mixer on medium speed until completely smooth and creamy, about 2-3 minutes. Pour the batter evenly into the prepared baking pan, spreading to corners.
03 - In a separate medium bowl, whisk together melted butter, brown sugar, ground cinnamon, and flour until a thick, spreadable paste forms. The mixture should have a consistency similar to cookie dough.
04 - Drop spoonfuls of the cinnamon mixture randomly over the surface of the cake batter. Using a butter knife or skewer, gently swirl through the batter in a figure-8 or circular motion to create a marbled ribbon effect. Be careful not to over-mix.
05 - Place the pan in the center of the oven and bake for 32-38 minutes, or until a wooden toothpick inserted into the center comes out clean with moist crumbs. The top should be golden brown and spring back when lightly touched.
06 - While the cake bakes, whisk together powdered sugar, 2 tablespoons of milk, and vanilla extract in a small bowl until smooth and pourable. Add additional milk 1 teaspoon at a time if needed to reach desired consistency.
07 - Remove the cake from the oven and let cool in the pan for 10-15 minutes. Drizzle the vanilla glaze evenly over the warm cake using a spoon or whisk. Slice into squares and serve while still warm for the best texture and flavor.